The replication and transcription of sex-factor DNA in E. coli will be studied using three different methods. (1) The effect of the various DNA temperature-sensitive mutants on the complementary sex- factor strand synthesis which occurs in recipient cells which have received a transferred sex-factor DNA strand will be determined. Preliminary experiments have indicated that the best way to carry out these studies will be to measure the yield of circular DNA when the DNA is transferred into irradiated temperature sensitive recipients (to reduce the replication of chromosomal DNA). (2) A physical map of the R-factor (R538) will be constructed by mapping the fragments produced by the R1 restriction endonuclease. After the fragment map has been constructed, we wilh follow the procedure used for SV-40 virus to determine the origin and direction of replication of the R-factor DNA molecule. (3) R-factor DNA transcription will be studied by determining the size and location of the regions of the R-factor which are being transcribed. This will be done by (A) treating the hybrids with S-1 nuclease to determine the size of the hybridized regions (B) mapping the hybrids by electron microscopy and (C) when the map of the R-1 fragments has been constructed, the individual fragments will be hybridized with RNA to produce a transcription map of the R-factor.